Local data saves on iOS (best practices)

I’ve been working on an iOS project based around using the GameCenter for turn based matches.
As it stands everytime a player leaves the match the matches current state (in our case object positions) are parsed to a byte array then serialized as XML.

Really saving to XML is probably completely unnecessary as the same class which saves this data also opens and parses it. What would be the best format to save the data for extremely fast retrieval and saving. A few ideas I had were to save the byte array to a raw text document.

Any ideas? C.

I agree with @Fattie in the comments, but if you want to convert a byte array into a string without the XML (not sure what that would be doing that was too slow) you can use

   var myString = Convert.ToBase64String(yourByteArray);

And to revert

    var myByteArray = Convert.FromBase64String(myString);